1. Field of the Invention
The present invention relates to a semiconductor laser device which has an active layer of a multi-quantum well structure.
2. Prior Art
A semiconductor laser device which has a quantum well structure as an active layer, i.e., as a light emitting region is known to oscillate at a lower threshold current and operates with higher power as compared with a semiconductor laser device having an active layer formed of a bulk semiconductor material.
An example of such a laser device is illustrated in FIG. 1.
The illustrated laser device has a buried hetero structure. Specifically, the laser device has a mesa-type layered structure which comprises an n-type semiconductor substrate 1, and an n-type lower cladding layer 2, a nondoped lower optical confinement layer 3, an active layer of quantum well structure 4, a nondoped upper optical confinement layer 5, p-type upper cladding layers 6A, 6B and a p-type cap layer, which are sequentially grown on the semiconductor substrate 1, for example, by a crystal growth method such as an MOCVD method.
Also, current blocking layers 8 are formed on both sides of the mesa-type layered structure by sequentially growing a p-type layer 8A and an n-type layer 8B. Then, an n-type electrode (not shown) is formed on the back surface of the substrate 1, and a p-type electrode (not shown) is formed on the cap layer 7, independently.
Here, when an InP substrate is used as the substrate 1, and GaInAsP-based materials are used as materials for the layers sequentially grown on the substrate 1, selenium (Se), sulfur (S) and so on are generally used as an n-type dopant for the n-type lower cladding layer 2, while Zinc (Zn) is generally used as a p-type dopant for the p-type upper cladding layers 6A, 6B. Also, the lower optical confinement layer 3 and the upper optical confinement layer 5 constitute a GRIN-SCH structure for increasing the optical confinement effect and efficiently injecting a current injected from the electrode to the active layer 4. For this reason, the laser device having the layered structure reduces the threshold current and increases the external differential quantum efficiency of the laser device, thereby realizing a high optical output power operation.
Such a semiconductor laser device is mounted on a cooler comprising, for example, a Peltier device and accommodated in a package to assemble a laser module. The assembled laser module is used as a signal light source for an optical communication system, and a pumping light source for an optical fiber amplifier such as an erbium-doped fiber amplifier (EDFA).
In recent years, with rapid development in constructing WDM systems, a laser module serving as a pumping light source for an optical fiber amplifier incorporated in the system is required to have the performance which enables laser light having high optical output power to be output from an output end of an optical fiber that is optically connected to the laser module.
For realizing higher optical output power at the output end of an optical fiber, it is effective to enable a laser device itself, incorporated in a laser module, to operate at high optical output power.
It is an object of the present invention to provide a semiconductor laser device having a novel layered structure which is capable of operating at higher optical output power as compared with the conventional laser device having the layered structure illustrated in FIG. 1.
To achieve the above object, the present invention provides a semiconductor laser device comprising:
a layered structure having an n-type lower cladding layer, a lower optical confinement layer, an active layer having a quantum well structure, an upper optical confinement layer, a Zn-doped p-type upper cladding layer, and a cap layer grown on an n-type semiconductor substrate in this order; and
a nondoped semiconductor layer interposed between the upper optical confinement layer and the Zn-doped p-type upper cladding layer.